The present invention relates generally to wireless communication devices, and in particular to a transmitter portion of a wireless communication device.
The frequency spectrum that is shared among radio communication devices is limited. Thus the ability of a transmitter to transmit as much information as possible in an allocated frequency spectrum or channel without interfering with other communication devices in adjacent channels is of great importance. To transmit as much information as possible in the allocated channel, digital communication systems typically modulate both the amplitude and phase of a radio frequency (RF) carrier. The amplitude modulation allows more information to be encoded on the carrier in a given channel than if only the phase was modulated. However, the amplitude modulation puts additional requirements on the transmitter that would not exist if only the phase of the RF carrier was modulated.
These additional requirements are due to the inherent nonlinear effects resulting from the amplification of an amplitude modulated signal by an RF power amplifier. Due to the nonlinear characteristics of the RF power amplifier, signal distortion components that include an amplitude component and a phase component are added to the original signal. These additional components are due to the amplitude compression characteristics (AM/AM) and the phase distortion (AM/PM) characteristics of the RF power amplifier when it is driven over a range of amplitudes. If these distortion components are not compensated they will cause spreading of the spectrum into the adjacent channels and thus interfere with communication devices using adjacent channels.
A number of prior art signal processing techniques have been developed to compensate for the nonlinear characteristics of RF power amplifiers. One such technique involves the use of a feed forward correction circuit in a feed forward amplifier. In general, feed forward amplifiers separate out distortion components generated by the RF power amplifier to create an error signal. The error signal is then amplified and added to the RF power amplifier""s output with an amplitude, phase, and delay adjusted for maximum cancellation of the distortion components. However, the amount of distortion reduction available in a feed forward amplifier is limited by the distortion introduced into the error signal when the error signal is amplified by an error amplifier.
For example, FIG. 1 is a block diagram of an exemplary feed forward amplifier 100 of the prior art. Feed forward amplifier 100 includes a main signal path 102, a feed forward correction circuit 104, and a control circuit 106. An input signal 101 having carrier components is sourced to main signal path 102, where the signal is routed to a gain and phase adjuster 110 via an input signal coupler 108. Gain and phase adjuster 110 adjusts the amplitude and phase of input signal 101 based on a control signal received from control circuit 106. Gain and phase adjuster 110 conveys the amplitude and phase adjusted input signal to a radio frequency (RF) power amplifier 112 that amplifies the signal to produce an amplified signal 113. RF power amplifier 112 then conveys amplified signal 113 to an output signal coupler 120 via a signal coupler 116 and a delay circuit 118. As mentioned above, RF power amplifier 112 introduces distortion components to the amplified signal, which distortion components are partially cancelled by an error signal output by feed forward correction circuit 104.
Feed forward correction circuit 104 produces the error signal based on input signal 101 and amplified signal 113. A summation junction 124 included in feed forward correction circuit 104 receives a portion of input signal 101 from input signal coupler 108 via delay circuit 122 and further receives a portion of amplified signal 113 from signal coupler 116. Summation junction 124 subtracts the received portion of the amplified signal from the received portion of the input signal to produce an error signal 125. The subtraction results in a partial cancellation of the carrier components of the received portion of amplified signal by the carrier components of the received portion of the input signal. As a result, error signal 125 primarily contains the distortion components of the received portion of the amplified signal.
Summation junction 124 then conveys error signal 125 to a feed forward correction circuit error amplifier 130 via a feed forward signal coupler 126 and a feed forward gain and phase adjuster 128. Error amplifier 130 amplifies the received error signal to produce an amplified error signal 131 and conveys the amplified error signal to output signal coupler 120. Output signal coupler 120 combines amplified error signal 131 with amplified signal 113 to partially cancel the distortion components of amplified signal 113 and produce a distortion reduced output signal 121.
Amplification of error signal 125 by error amplifier 130 may result in an introduction of distortion components to the error signal due to the amplitude compression and the phase distortion characteristics of the error amplifier. Since amplified error signal 131 is combined with amplified signal 113 at output signal coupler 120, it is desirable to minimize the added distortion. In order to reduce the distortion introduced into amplified error signal 131 by error amplifier 130, control circuit 106 controls an average power of an error amplifier drive signal, that is, error signal 125.
Control circuit 106 receives a portion of error signal 125, that is, attenuated error signal 127, from feed forward signal coupler 126. Attenuated error signal 127 is routed to an average power detector 132, which determines an average power of the attenuated error signal. A controller 134 coupled to power detector 132 reads the average power determined by detector 132 and, based on the average power, conveys a control signal to gain and phase adjuster 110 that minimizes the average power detected by detector 132. By controlling the amplitude and phase adjustment of input signal 101 by gain and phase adjuster 110, control circuit 106 exerts control over an average power of error signal 125 and thereby over an average power of the drive signal applied to error amplifier 130.
However, control of an average power of error signal 125 does not necessarily minimize the distortion introduced into the amplified error signal by error amplifier 130. By controlling the average power of the error amplifier drive signal, the prior art provides sub-optimal reduction of distortion introduced by feed forward correction circuit 104 as the prior art fails to minimize the peak power of the error amplifier drive signal. Therefore a need exists for a method and apparatus for minimizing the distortion introduced into a feed forward amplifier by the feed forward correction circuit.